Nervous System, Sensory Organs & Action Potentials

Neuron Vocabulary: Synapse:

Synapse: represents the junction where one neuron communicates with another neuron. Neurotransmitters diffuse across here.

Neuron Vocabulary: Synaptic Cleft

Synaptic Cleft: the gap between the presynaptic and postsynaptic membrane

Defintion: Summation

Temporal Summation: involves single synapse- ESPS's occur one after another Spatial Summation: involves multiple synapses- ESPS's occur at the same time

Limbic System

The limbic system is a set of structures in the brain that deal with emotions and memory. It regulates autonomic or endocrine function in response to emotional stimuli and also is involved in reinforcing behavior. Besides the hypothalamus, the amygdala, hippocampus (long term memory), and thalamus all make up this limbic system. This system is involved with emotions and plays a role in memory.

Part of the Brain: Hypothalamus

The link between the endocrine and the nervous system. (A favorite test question from teachers!) The nervous system responds quicker, in general to stimuli than the endocrine system. The endocrine system secretes hormones that travel to target organs, while the nervous system uses neurotransmitters and action potentials to respond to stimuli. The hypothalamus is involved with a range of functions including fluid balance, blood pressure, body temperature, and maintaining homeostasis. The hypothalamus is also involved with emotions, sex drive, thirst, and even the production of digestive juices.

The nodes of Ranvier

The nodes of Ranvier are the gaps between adjacent Schwann cells. At these nodes, the axolemma is especially permeable to Na+ and K+. These nodes are only present when the axon is myelinated. Bottom line: these gaps are rich in ion channels!!

Ok... now what? We have just seen that depolarization has occurred, a voltage has been generated and is moving down the axon... now what?

The presynaptic neuron has vesicles that are filled with neurotransmitters. Gated channels for Ca++ are also present. As the action potential arrives, Ca++ voltage-gated channels open... allowing Ca++ to diffuse into the neuron. This Ca++ influx causes the synaptic vesicles to fuse with the presynaptic membrane. These vesicles dump their neurotransmitters into the synaptic cleft. The neurotransmitter can now bind to the receptor located on the postsynaptic cell's membrane. Binding will change the shape of the receptor... this allows a "passage" or entry area for ions to now cross. For example, the passage can allow Na+ and K+ ions to cross.

Action Potential: Refractory Period/Hyper polarization (4) 1) Absolute Refractory period: no stimulus, no matter how strong can elicit another action potential 2) Relative Refractory period: a very strong stimulus would be needed to elicit an action potential

The rapid loss of K+ ions (and influx of Cl-) causes the membrane potential to overshoot its resting potential of -70mv and reach a membrane potential of -98mv The sodium potassium pumps help return the neuron back to its normal membrane potential (pumps 3 Na+ out and 2 K+ into the cell) The neuron cannot fire during this period until it reaches its resting state again (-70 mv)

Cells of the Nervous System: microglia

These are phagocytic cells located in the CNS. They account for 5% of the glial cells.

Enteric Nervous System

This extends the entire length of the alimentary tract... from the esophagus to the anus. It controls the secretory and motile functions of our digestive tract. The older literature did not include this system as a division of the autonomic nervous system, but it has become added. The number of neurons associated with the enteric system is enormous. The gut contains 100 million neurons, more than even the spinal cord!! The enteric nervous system has been called "The Second Brain"- it can work alone, independent of sympathetic and parasympathetic systems, but can work with them as well. This system makes over 30 neurotransmitters. 90% of our serotonin is made in the gastrointestinal tract, and a reasonable % of dopamine too!

Action Potentials

This is a nerve impulse. It is all-or-none. A stimulus opens the voltage-gated Na+ channels which cause Na+ to flow into the neuron. This brings the potential to a positive value. As this occurs, we see a large change in the membrane voltage... this is an action potential. Negatively Charged Proteins and Cl- ions help balance out the influx of positive ions

Parts of the Brain: Cerebellum

This part of the brain is involved with balance and muscle coordination. If there is an injury to the cerebellum, fine movement, equilibrium, posture, and motor learning would all be affected. The basal ganglia which includes the caudate nucleus, putamen, and globus pallidus work with the cerebellum to coordinate fine motor movements such as moving your fingertips.

Middle Ear

Vibrations are transmitted to the three bones: incus, malleus, and stapes. These bones actually form a "bridge" connecting the tympanic membrane to the inner ear. Vibrations move to the oval window before moving to the inner ear. The oval window is a membrane that causes vibrations to move to the inner ear. The eustachian tube connects the middle ear to the nasopharynx and ensures that air pressure is equalized on either side of the eardrum. Did your ears ever "pop" when changing altitude in a plane? The eustachian tube is what enables you to equalize the pressure difference and restore your hearing to normal!!

Action Potential: Depolarization (2)

Voltage gated sodium channels open and Na+ rushes into the cell along the axon making the membrane potential even more positive causing more ion channels to open so more Na+ can enter the cell This occurs until the membrane potential reaches equilibrium for sodium ions (around 58 mv)

Spinal Cord (roots)

a) Dorsal root: transmits sensory information "sensory root" b) Ventral root: this is the "motor root" and it carries information towards the limbs and organs

CNS

brain and spinal cord

Lobes of the Brain: Frontal

frontal lobe controls motor function

Blood-Brain Barrier

high density cells (endothelial cells) that prevents the passage of substances such as pathogens, antibiotics, and chemicals. Some molecules such as CO2, O2, water, glucose, and amino acids can cross. Astrocytes have long been given credit for the creation of this barrier, but I have been reading conflicting data that suggest this might not be true.

PNS

located outside the CNS, consists of cranial nerves, spinal nerves, and associated ganglia

Remember: speed is constant along the axon

speed is constant along the axon... First, let's talk about speed. The larger the diameter, the faster the impulse can move along the axon! If you increase the stimulus, we generate more action potentials but the speed and amplitude (height) of the action potential will not change. The signal is conducted from the axon of a presynaptic cell to the dendrite of a postsynaptic cell. Communication occurs at special junctions called synapses... and the message is unidirectional... in that in can travel only one way.

PNS can further be divided into: 1) Somatic nervous system 2) Autonomic nervous system

1) Somatic nervous system: voluntary... will carry impulses to skeletal muscles, tendons, and skin 2) Autonomic nervous system: involuntary... impulses are transmitted to cardiac muscle, smooth muscle, or glands. This can further be divided into: Sympathetic: think energy generation... fight-or-flight Parasympathetic: think calmness... rest-and-digest

The Brain Basics

2 hemispheres are connected by a deep bridge of nerve fibers called the corpus callosum. Each hemisphere has an outer covering of gray matter called the cerebral cortex. Grey matter consists mainly of unmyelinated axons Gray matter contains cell bodies, glial cells, dendrites, and unmyelinated portions of axons. White matter consists mainly of myelinated axons. Underneath the gray matter lies the myelinated white matter: White matter connects various areas of gray matter to each other and carry impulses between neurons. Bundles of nerve fibers are called tracts. Tracts are used to describe the myelinated axons of the white matter. The corpus callosum is a white matter tract connecting the two brain hemispheres. It is the largest white matter structure in the brain. Bottom line: white matter is a relay and communication region of the brain.

Parts of the Brain: Thalamus

A "relay station" in the brain. It relays sensory information to the proper areas of the brain. The cerebrospinal fluid bathes, nourishes, and protects the brain and spinal cord from injury. The choroid plexus is a network of cells that produce the cerebrospinal fluid in the ventricles of the brain. The plexus consists of cells called ependymal cells. The plexus also is involved with removing waste, excess neurotransmitter, and foreign substances from the cerebrospinal fluid.

Neurotransmitters can include such molecules as: (Neurotransmitters can be excitatory or can be inhibitory)

Acetylcholine: many functions including learning and memory enhancement Dopamine: many functions including keeping focused and paying attention; promotes feelings of pleasure Serotonin: depression can result if you have a deficiency! Norepinephrine: involved with mental alertness and memory. (Norepinephrine and Epinephrine are both a hormone and a neurotransmitter) GABA: body's inhibitory neurotransmitter. Keeps you calm and without anxiety. Alcohol is thought to mimic the effect of GABA in the brain, decreasing anxiety and inhibitions. Glycine: works with GABA to help decrease anxiety among other functions. Many have overlapping functions and is still being researched

Eye Fluids: Aqueous humor

Aqueous humor: clear, slightly alkaline fluid found in anterior and posterior eye chambers. It resembles blood plasma, but has less protein and glucose. It is involved with water removal, and it brings O2, and nutrients to the eye.

Reflexes

Are automatic, unconscious responses to certain stimuli A reflex arc pathway involves: a) Receptor... e.g. dendrite b) Sensory neuron... relays impulse to CNS c) Reflex center (composed of interneurons) ... arc is made d) Motor neuron... conducts impulse to the effector e) Effector... a muscle or gland For example, you touch a hot stove... pain receptors in the skin are stimulated and the message is sent to the sensory neuron. The message is then sent to the spinal cord... no conscious decision by the brain is needed...there at the reflex center the interneurons transmit impulses to the motor neuron, and then to the muscles (effector). The muscles contract and you withdraw your hand.

Cells of the Nervous System: Astrocytes

Astrocytes: Star-shaped cells, most numerous glial cells. Many functions include regulating electrical impulses, provide neurons with lactate for nutrients, contain channels for K+ transport, regulates blood flow, and even take part in neuronal repair.

Action Potential: Repolization (3)

At this point there is a large number of positively charged ions in the cell and potassium gated ion channels have opened and the K+ ions go down there concentration gradient and leave the cell This causes the charge in the cell to decrease (repolarization)

Definition: Axon

Axon: Extension of a neuron that carries nerve impulses away from the cell body to target cells such as nerve, muscle, or gland cells. No neuron has more than a single axon.

Retina: the posterior portion of the eye that contains the photoreceptors: Rods: for dim light and black- and- white vision Cones: provide color vision, respond well to daylight color and responsible for visual acuity Rods and cones found in the retina vary from animal to animal.

Cones are concentrated in a depression near the center of the retina called the fovea centralis... this is the area of "Keenest Vision" and a common exam question!! The Fovea has no rods.

Inner Ear

Consists of fluid-filled channels in the bones of the skull. One of those channels is called the cochlea. This is a snail-like structure that contains a sensory receptor called the organ of Corti. This organ of Corti holds the hair cells, the nerve receptors for hearing... the mechanoreceptors of the ear!! The movement of middle ear bones exerts pressure on the cochlea. The fluid in the cochlea stimulates tiny hair cells. The signals from these hair cells are changed into nerve impulses... Impulses are carried to the brain by the auditory nerve. Also in the inner ear are the semicircular canals, the utricle, and saccule. These structures are involved with balance. A common exam question!!

Cells of the Nervous System: Dendrite

Dendrite: highly branched neuronal extensions that bring nerve impulses toward the cell body, also called the soma Many mitochondria can be seen scattered in the cytoplasm of the soma, dendrites, and axon, but most abundant at the axon terminals. The axon terminals contain the neurotransmitters and get released at the synapse.

Endorphins

Endorphins are the natural body-analgesics that decrease our perception of pain. They are also neuropeptides. Besides decreasing our perception to pain, they may produce a euphoric or happy feeling in us.

Definition: Ganglion

Ganglion: a mass of neuron cell bodies usually found outside the CNS

Cells of the Nervous System: Glial cells

Glial cells: surround neurons and provide support and protection for them. Glial cells represent the most abundant cell in the CNS. They include astrocytes, oligodendrocytes, Schwann cells, microglia, satellite cells, and ependymal cells.

Parts of the Brain: Medulla Oblongata

Helps regulate functions such as breathing, heart rate, swallowing, and digestion The "center for respiration and circulation" Involved in involuntary functions •Contains myelinated and nonmyelinated fibers Cardiac, respiratory, and even sneezing and vomiting centers are located here. •Chemoreceptors can tell if the blood is acidic or not. Chemoreceptors send signals to the medulla which can respond with increasing the breathing rate, for example.

Neuron Types: Interneuron

Interneuron: located between sensory and motor neuron; integrates sensory input and motor output

Neuron Types: Motor Neuron (efferent)

Motor (efferent): carry impulses out from the brain or spinal cord to effectors such as muscles or glands Most nerves have both sensory and motor fibers and are called mixed nerves.

Pathologies

Multiple Sclerosis is considered an auto-immune disease. NMR data has shown a break-down of the blood brain barrier in a section of the CNS... allowing T-Lymphocytes to cross over and attack the myelin sheath. Thus, MS is also a disease of the blood-brain barrier. In epilepsy, we see uncontrolled seizures. Studies have shown, there is blood brain barrier "failure" that leads to seizures.

Definition: Neuron

Neuron: the basic unit of nervous tissue, it is a nerve cell including cell body, axon, and dendrites They are so specialized, thus incapable of reproducing themselves. Replacing old neurons with new neurons would literally "erase" our memory away... thus no mitosis with these cells. Their ability for regeneration is very poor.

Cells of the Nervous System: Oligodendrocytes

Oligodendrocytes: makes myelin in the CNS

External Ear

Pinna: your outer ear you touch Auditory canal... collects the sound waves that enter the ear. These waves are channeled to the cone-shaped eardrum (tympanic membrane). The tympanic membrane vibrates in response to the sound wave.

Neuron Vocabulary: Postsynaptic Cell:

Postsynaptic Cell: the cell that has the receptors that bind to specific neurotransmitters

Neuron Vocabulary: Presynaptic Cell:

Presynaptic Cell: this cell stores the neurotransmitters such as acetylcholine stored in vesicles. Can be a muscle, neuron, or even a gland.

Tongue Anatomy and Taste

Receptor cells are organized into modified epithelial cells called taste buds. Taste buds are seen in several areas; most are associated with nipple-shaped projections called papillae. About 3000 taste buds are on the tongue. There are 4 primary taste sensations: sweet, sour, salty, and bitter. Taste perception is a tricky affair; it also involves the smell receptors. This is evidenced by the decreased taste ability of people who get colds and are congested... We can't taste very much! Von Ebner's glands surround the papillae and secrete salivary lipase, beginning the process of lipid breakdown (hydrolysis) in the mouth.

Action Potential: Threshold and Membrane potential (1)

Resting potential of a neuron cell is around (-70mV) If a stimulus does cause the cell to achieve a specific voltage (((around -55mV))) nothing will happen. Once the required voltage is achieved, the neuron generates an action potential Once the neuron "fires", the size of the action potential does not depend on the strength of the stimulus causing depolarization.

Rhodopsin Protein

Rhodopsin is a biological pigment found in the rods of the retina and is a G-protein-coupled receptor (GPCR). It belongs to opsins. Rhodopsin is extremely sensitive to light, and thus enables vision in low-light conditions. When rhodopsin is exposed to light, it immediately photobleaches.

Cells of the Nervous System: Schwann Cell

Schwann Cell: makes myelin in the PNS

Eye Anatomy

Sclera: white area of the eye... tough connective tissue made up of collagen bundles, it is avascular, but has sensory receptors for pain. Cornea: allows light into the eye; avascular when light strikes the cornea, it bends (refracts) the incoming light onto the lens of the eye. Choroid: highly vascular thin layer that lines most of the internal sclera surface. Melanocytes containing pigments are here which can absorb the light that photoreceptors have not and prevents light scattering within the eye. Iris: colored portion of the eye and is continuous with the choroid Pupil: opening in the iris center that controls the amount of light that enters. The size of the pupil determines how much light enters the eye. Sympathetic stimulation: dilates pupil, Parasympathetic stimulation: constricts pupil Lens: located behind the pupil, it will focus light onto the retina, the ciliary muscle will change the shape of the lens

Neuron Types: Sensory Neuron (afferent)

Sensory (afferent): closely associated with receptor cells; responds to external stimuli or internal conditions and relays them to brain and spinal cord